The present invention relates to U.V. laser surface treatment methods, more particularly to removal of any foreign materials from substrate surfaces. For example, such a treatment is the complete stripping of photoresists or the removal of any foreign materials, including those on side walls, formed during Reactive Ion Etching (RIE) or High Dose Ion Implantation (HDI) processes, common in the VLSI/ULSI (Very/Ultra Large Scale Integration) circuits industry. The invention also includes the removal of particles down to sub-micron sizes and atomic contaminants, such as heavy metals and alkaline elements, from substrate surfaces.
In the manufacturing of various products it is necessary to apply a layer of protective material on a surface, which must be removed after a specified manufacturing step has been concluded. An example of such process is the so-called xe2x80x9cphotolithographyxe2x80x9d process, which is widely used in the manufacturing of integrated circuits. In this process, a pattern is created on a surface using a layer of protective material illuminated through a mask, and the surface is then treated with a developer which removes material from the unmasked portions of the surface, therefore leaving a predetermined pattern. The surface is then treated by ion implantation or by etching agents, which introduce the implanted species into, or removes material from, the unmasked portions of the surface. Once these processes are completed, the role of the protecting mask ends and it must be removed. The process is conventional and well known in the art, and is described, e.g., in R. K. Watts, xe2x80x9cLithographyxe2x80x9d, VLSI/ULSI Technology, S. M. Sze, ed., McGraw-Hill, New York, 1988, Chpt. 4.
Two main photoresist stripping methods are known in the modern VLSI/ULSI circuits industry (see D. L. Flamm, xe2x80x9cDry PlasmaResist Strippingxe2x80x9d, Parts 1, 2 and 3; Solid State Technology, August, September and October 1992):
1) Wet stripping which uses acids or organic solvents;
2) Dry stripping, which uses plasma, O3, O3/N2O or UV/O3-based stripping.
Both methods are problematic and far from being complete, especially when taking into consideration the future miniaturization in the VLSI/ULSI industry. The current technology is capable of dealing with devices having feature sizes of about 0.5 xcexcm, but the workable size of the devices is expected to be reduced before the end of the century to 0.25 xcexcm. The expected size changes require considerable changes in the manufacturing technology, particularly in the stripping stage. The prior art photoresist stripping techniques, described above, will be unsuitable for future devices, as explained hereinafter.
Utilizing only the wet stripping method is not a perfect solution, as it cannot completely strip photoresist after tough processes that change the chemical and physical properties of the photoresist in a way that it makes its removal very difficult. Such processes include, e.g., High Dose Implantation (HDI), reactive Ion Etching (RIE), deep UV curing and high temperatures post-bake. After HDI or RIE, the side walls of the implanted patterns or of the etched walls are the most difficult to remove.
In addition, the wet method has some other problems: the strength of stripping solutions changes with time, the accumulated contamination in solutions can be a source of particles which adversely affect the performance of the wafer, the corrosive and toxic content of stripping chemicals imposes high handling and disposal costs, and liquid phase surface tension and mass transport tend to make photoresist removal uneven and difficult.
The dry method also suffers from some major drawbacks, especially from metallic and particulate contamination, damage due to plasma: charges, currents, electric fields and plasma-induced UV radiation, as well as temperature-induced damage, and, last but not least, it suffers from incomplete removal. During various fabrication stages, as discussed above, the photoresist undergoes chemical and physical changes which harden it, and this makes the stripping processes of the prior art extremely difficult to carry out. Usually a plurality of sequential steps, involving wet and dry processes are needed to remove completely the photoresist.
The art has addressed this problem in many ways, and commercial photoresist dry removal apparatus is available, which uses different technologies. For instance, UV ozone ashers are sold, e.g. by Hitachi, Japan (UA-3150A). Dry chemical ashers are also available, e.g., by Fusion Semiconductor Systems, U.S.A., which utilize nitrous oxide and ozone to remove the photoresist by chemical ashing at elevated temperatures. Microwave plasma ashing is also effected, e.g., as in the UNA-200 Asher (ULVAC Japan Ltd.). Also plasma photoresist removal is employed and is commercially available, e.g., as in the Aspen apparatus (Mattson Technology, U.S.A.), and in the AURA 200 (GASONICS IPC, U.S.A.).
More recently, photoresist removal has been achieved by ablation, using laser UV radiation, in an oxidizing environment, as described in U.S. Pat. No. 5,114,834. The ablation is due to strong absorption of the laser pulse energy by the photoresist. This process is a localized ejection of the photoresist layer to the ambient gas, associated with a blast wave due to the breaking of chemical bonds in the photoresist and instant heating. The partly gasified and partly fragmented photoresist is blown upwards away from the surface, and instantly heats the ambient gas. Fast combustion of the ablation products occurs due to the blast wave, and may also be due to the photochemical reaction of the IN laser radiation and the process gases. The main essence of the process is laser ablation with combustion of the ablated photoresist, which occurs in a reactive gas flowing through an irradiation zone. The combination of laser radiation and fast combustion provides simultaneous lowering of the ablation threshold of hard parts of the photoresist (side walls). The combusted ablation products are then removed by vacuum suction, or by gas sweeping leaving a completely clean surface.
While U.S. Pat. No. 5,114,834 provides an important novel process, it still does not provide a high throughput, which is industrially convenient, viz., an industrially acceptable number of wafers that can be stripped during a given time. The laser stripping throughput is determined by the stripping rate or by the number of laser pulses needed for providing complete stripping of a unit area of the photoresist per unit of time.
International Patent Application No. PCT/IL96/00138, published under the number WO 97/17166, the entire content of which is incorporated herein by reference, discloses a method of accelerating a laser stripping process carried out in a reactive gas mixture, comprising carrying out the stripping process in the presence of an accelerating effective amount of NxOy gas, preferably selected from among N2O, NO, NO2, N2O3, N2O4 and NO3. The reactive gas mixture may include oxygen or ozone. While oxygen based chemistry provides a good solution for assisting laser removal of organic materials, it does not give a complete answer for removing inorganic based residues with ablation threshold which exceeds the damage threshold of the substrate. Such residues are formed, for example, on side walls during RIE processes as in etching of metal, polysilicon, contacts, and via holes. The oxygen based gas composition described in WO 97/17166 gives atomic oxygen radicals. These radicals cannot break the strong chemical bonds in inorganic materials, as e.g. Sixe2x80x94O, Sixe2x80x94C, Sixe2x80x94Cl or other metal-oxygen bonds. Therefore the removal of the above mentioned residues containing such bonds is one of the challenges in dry stripping. Another challenge is the removal of submicron particles, as well as metallic contaminants, from silicon wafers or from other substrates. This is extremely important for ULSI technology.
While reference will be made throughout this specification to laser removal of foreign materials from semiconductor wafers, this will be done for the sake of simplicity, and because it represents a well known and widely approached problem. It should be understood, however, that the invention described hereinafter is by no means limited to laser removal of foreign materials from wafers, but it applies, nutatis mutandis, to many other applications, such as cleaning of foreign materials from different objects such as Flat Panel Displays (FPD), lenses, photo-masks, compact disks, magnetic heads etc.
It has now surprisingly been found, that addition of gases, the molecules of which contain fluorine and/or chlorine atoms, to the oxygen-based process gases used for laser removal of foreign materials, including particles and atomic contaminants, such as heavy metals, alkaline elements and atomic elements, in the method described in the aforementioned International Publication No. WO 97/17166xe2x80x94which gases contain at least a gas having the formula NxOy, viz. one or more nitrogen oxides, wherein x and y having the appropriate values for the given oxide or mixtures of the oxides, preferably selected from among N2O, NO, NO2, N2O3, N2O4 and NO3, permits the fast and complete removal of inorganic in addition to the organic foreign materials, for example removal of adhesion promoters and/or antireflection coatings used prior to photoresist application. This effect was also proved even for submicron geometries common in modern and future ULSI industry.
Accordingly, the method of accelerating a Laser Removal Process (LRP) carried out in reactive gas mixture, according to the invention, comprises carrying out the stripping process in the presence of an oxygen based gas and of a gas containing fluorine and/or chlorine in its molecule.
At elevated values of laser energy fluence and/or auxiliary gas irradiation by VUV source (vacuum UV, wavelength lower than 180 nm) one can choose an oxygen-based gas consisting of oxygen, ozone, nitrogen oxide (NxOy) or their mixtures, and a gas containing F and/or Cl atoms in its molecule from among NF3, SF6,SF4, CF4, CF3, NOCl, Cl2O, F2O, HF, F2, Cl2, HCl.
The most preferred gas containing fluorine and/or chlorine atoms in its molecule is nitrogen trifluoride (NF3), due to its lower thermo-dissociation energy. This gas promotes fast and complete laser removal of inorganic foreign materials, including micron and submicron particles and metallic contaminants, as well as preventing any damage to the treated substrates.